1. Field of the Invention
This invention relates to an apparatus for measuring the surface potential of electrostatically charged elements.
2. Description of the Prior Art
It is known in the art that the surface potential of an electrostatically charged element can be measured by placing a conducting probe at a fixed distance from the element and connecting the probe, with a capacitor to ground. Under these circumstances the relationship between the voltage at the surface under measurement (V.sub.s), the voltage at the probe (V.sub.p) and the voltage at the capacitor (V.sub.c), is ##EQU1## WHERE C probe &lt; &lt; C photoconductor.
By measuring the voltage on the capacitor (V.sub.c) with a conventional electrometer, the voltage on the charged surface (V.sub.s) can be determined. One practical limitation of the above-described measuring apparatus is that the resistance (R) of the electrometer will discharge the capacitor (C) so that the time constant (T=RC) must be long enough to provide desired accuracy with reasonable reading times. For example, if the desired accuracy is 1 percent and a time of 1 second is sufficient to read the meter, the time constant (T) must be 100 seconds. This normally dictates extremely large values for the resistor (R). Of course, the time constant (T) can also be increased by increasing the capacitor (C), but this is at the expense of V.sub.p.
One known technique to reduce the time constant required for accurate reading of the electrometer is to periodically chop the signal on the probe. By alternately exposing the probe to the electric field of the surface under measurement and grounding the probe, an AC voltage is built up on the capacitor which can be peak detected to provide a DC voltage representative of the voltage on the charged surface. The shorter time constant required allows the use of capacitance and impedance values that can be obtained through the use of conventional components. A voltage measuring apparatus using the above-described signal chopping technique is disclosed in U.S. Pat. No. 3,729,675 to Vosteen issued Apr. 24, 1973. To provide the periodic grounding of the probe, the Vosteen apparatus includes a grounded band attached to the surface to be measured. One disadvantage of this arrangement is that the placement of a grounded band on the surface of a charged element in situ in a machine is frequently a difficult operation. To overcome this difficulty, the element can be removed from the machine for measurement on a separate apparatus or, alternatively, the grounded band can be included in the manufacture of the element itself. The first alternative is time consuming and, more importantly, eliminates environmental changes in the machine which affect charging of the element. The second alternative increases the cost and complexity of the element. Furthermore, a charged element with an integral grounded band cannot be used in certain applications. For example, in electrophotographic copying machines, a grounded band attached to the photoconductive imaging surface would (1) adversely affect the triboelectric relationship of the developer mixture; (2) attract the receiver sheet used in transfer electrophotography so that the sheet will not separate from the photoconductive surface; and (3) short circuit an electrically biased development electrode as the grounded band moved past such electrode. For these and other reasons, the development of a voltage measurement apparatus capable of in situ operation in a machine continues to be a problem in the art.